(CNN) A government watchdog is raising new concerns about the safety and readiness of US ships in the Pacific region following a series of collisions that led to the deaths of US sailors.
More than a third of warfare training certifications for the Navy's Japan-based destroyers and cruisers had expired in June -- a five-fold increase from two years prior, the Government Accountability Office has found.
In testimony to be delivered Thursday, the GAO found that 37% of the Navy's destroyers and cruisers in Japan had expired -- up from 7% in January 2015 -- and over two-thirds had been expired for five months or more.
The GAO testimony, a copy of which was obtained by CNN, will fuel new questions on Capitol Hill about the cause of the four Navy ship collisions in the Pacific this year, in which 17 sailors were killed, and the health of the Navy fleet.
After the USS John S. McCain collided with a merchant vessel last month off the coast of Singapore -- an incident that accounted for 10 of the 17 deaths -- the commander of the 7th Fleet was dismissed, and the Navy ordered a rare, one-day operational pause for its entire fleet.
Vice Chief of Naval Operations Adm. Bill Moran, who ordered a review of surface fleet operations after the McCain collision, is testifying Thursday before the House armed services committee on the USS McCain and USS Fitzgerald collisions.
There are 70 to 80 ships and submarines in the 7th Fleet, which is on the front line of sea-based missile defense against North Korea. The GAO testimony focused on the destroyers and cruisers, the two kinds of ships involved in the four collisions this year.
The Navy's ships require more than a dozen training certifications, including mobility and seamanship and warfare capabilities like ballistic missile defense and surface warfare.
The cause of the McCain collision is still under investigation, but military leaders, lawmakers and the GAO have long warned about the Navy's readiness crunch as the size of the fleet has increased and the number of ships deployed has remained constant, while the length of deployments has increased.
"The Navy has had to shorten, eliminate, or defer training and maintenance periods to support these high deployment rates," John Pendleton, director of the GAO defense capabilities and management, said in the written testimony.
But the GAO has also issued specific warnings about ships based abroad, and specifically Japan.
In a May 2015 report, the GAO said that the Navy's schedules for overseas ships limited dedicated training and maintenance time -- and found that incidents of degraded or out-of-service equipment nearly doubled from 2009 to 2014.
"Navy officials told us that US-based crews are completely qualified and certified prior to deploying from their US homeports, with few exceptions," the testimony states. "In contrast, the high operational tempo of ships homeported overseas had resulted in what Navy personnel called a 'train on the margins' approach, a shorthand way to say there was no dedicated training time set aside for the ships so crews trained while underway or in the limited time between underway periods."
The 2015 report cited Japan-based ships in particular for not having any dedicated training time -- an issue the GAO says is still not resolved.
"The Navy has made plans to revise operational schedules to provide dedicated training time for overseas-based ships, but this schedule has not yet been implemented," the testimony states.
Still, the Navy's training certification issues may raise the sharpest questions from lawmakers.
As of June 2017, the GAO found that the eight of the Navy's 11 destroyers and cruisers based in Japan had expired certifications for mobility and seamanship, air warfare and undersea warfare, according to a GAO analysis obtained by CNN.
The most common reason cited for expired certifications was the operational schedule of the ships, according to the GAO analysis.